Viper may not have noticed Arty leaving the group, but Jack didn’t notice her coming back, either. Instead he was busy zoning out staring at a footprint (one of their own, even. Very useful to the investigation).
But he did belatedly register his name and looked up in time to see the quarter flashing. His head snapped sideways as he tracked the thing’s trajectory before turning back to blink at Viper again.

“Ya coulda just asked nicely.” If, y’know, he was remotely mentally present to hear you asking nicely. Sorry, Arty.
He briefly checked his bag to make sure everything was safe before shucking his hoodie onto the sand and diving out there with everything else still on. See you space cowboy…
After going out a ways—probably someone taller could have kept their feet on the sand still, but alas, no humans here fit that description—and vanishing under the water for a bit, he returned. Hands behind his back, he slogged his way over the last of the waves and sand trying to trip him up.

“Good news! You didn’t scare all the fish off after all!”
He flung his arms to the front to point a gun at Viper’s face and fire.
…
Or at least, pull the trigger with a little click. Nothing actually came out.